This invention relates to a method for forming a fine thick film pattern, where the desired circuit pattern is formed by coating a circuit-forming thick film paste on an insulating film substrate, and to a method for economically and easily forming a fine thick film pattern for a very fine circuit, which is particularly effective when used in thick film hybrid ICs and plasma dislay units, etc.
Hybrid ICs are categorized as thick film hybrid ICs or as thin film hybrid ICs in accordance with the film forming method employed.
In the case of a thick film hybrid IC, a circuit film is formed on the substrate using a paste for forming various kinds of circuits. Various kinds of pastes can be obtained in the market, namely, conductor pastes (which can be obtained by mixing a noble metal powder such as Au, Ag, Au-Pt, etc., and glass frit and dispersing this mixture into an organic resin binder solution), resistance pastes (which can be obtained by mixing a high-resistance powder such as Ag-Pd or ruthenium oxide and glass frit and dispersing this mixture into an organic resin binder solution), dielectric pastes (which can be obtained by mixing the powder of a ferrodielectric material such as titanic acid or zircon acid, etc., and glass frit and dispersing this mixture into an organic resin binder solution), or insulating material pastes (which can be obtained by dispersing glass frit into an organic resin binder solution). Currently, mixtures of various kinds of pastes are available from a variety of paste makers. The pastes generally used employ an organic resin binder solution having a certain degree of viscosity. Such binder solutions can be obtained by dissolving an organic resin such as methyl-cellulose, ethyl-cellulose, etc., into an organic solvent having a high boiling point, such as butyl-carbithol or butyl-calbitholacetate, etc.
In the existing thick film pattern forming method, conductor patterns, resistor patterns or capacitor patterns of the desired form are obtained by coating one of the above-mentioned pastes on a ceramic insulated substrate by the screen printing method. Thereafter, wiring conductors, resistors or capacitors are formed on the insulating substrate by baking said circuit pattern.
Thin film hybrid ICs may be formed by using a sputtering system or an evaporation system. A metal such as Au, Al, Ni-Cr, Ta, etc., is evaporated by said system and coated onto the entire surface of an insulated substrate. Thereafter, the unwanted portions of the metallic thin film are eliminated by the photoetching process, and wiring conductors, resistors and capacitors of the desired form can be formed on the insulated substrate by patterning.
The system for forming a paste film on an insulated substrate in the above-mentioned thick film forming method is very economical as compared with the thin film forming method, and requires a relatively short time for film formation with simple controls during formation.
Thus, the conventional thick film method allows low cost circuit substrates to be produced on a mass-production line. But on the other hand, the fine patterns which can be obtained by the thin film forming method cannot be obtained by the conventional thick film forming method.
The existing thick film forming method is limited by the size of the screen mesh, as explained above, because the circuit pattern is formed by screen printing and the pattern width in practice is generally limited at most to 200 .mu.m, As a result, it is difficult to obtain a pattern having a pattern width of 50 .mu.m, which can be obtained by the thin film forming method.
For this reason, conventional hybrid ICs having high density electrical circuits are produced by the thin film forming method even if they do not require formation of electrical circuits with high accuracy from the electrical viewpoint. This is very expensive.
In addition, as a special example of the thick film forming method, photo-etching technology can be employed in order to obtain a fine circuit pattern. In this method, a paste containing photo-sensitive material is coated on the insulated substrate and dried. Then, this paste film itself is exposed and developed in order to form the desired circuit pattern. Thereafter, said circuit pattern is baked and a circuit board is thereby obtained.
However, this method is expensive because the paste is obtained by a special production method. Moreover, some film elements cannot be obtained because the commercially available pastes are limited to such types as Au conductive paste or Ag conductive paste, etc. Application of these special pastes complicates handling and also makes it difficult to obtain a homogeneous fine pattern with excellent reproduceability, and examples of practical applications of this thick film method appear to be very rare.
Moreover in plasma displays, which have two sheets of glass substrates respectively providing the X direction electrode pattern and the Y direction electrode pattern and a gap between said glass substrates filled with a rare gas such as neon or helium, etc., the economical self-shift type panel requires a pattern as fine as 60 .mu.m because said X and Y electrode patterns frequently must be formed in complicated meandering patterns. The pattern formation obtained by the above-mentioned thin film forming method results in relatively high cost.